Schottky diodes, also known as Schottky-barrier diodes, are well known devices that can be used in a number of applications, including rectification in power supplies and voltage clamping. Recently, Schottky diodes comprising layers of GaN and AlGaN provided on a semiconductor substrate have drawn a lot of attention in view of their potential for replacing conventional silicon (Si) or silicon carbide (SiC) based devices for high voltage applications.
A Schottky diode generally includes of a Schottky contact (the anode) and an ohmic contact (the cathode) that are both provided on a semiconductor substrate having one or more layers. One example of a semiconductor device comprising a Schottky diode is shown in FIGS. 1A and 1B. Note that FIG. 1A is a side view of the device, while FIG. 1B is a top view.
The device in FIGS. 1A and 1B includes a semiconductor substrate 20 including a plurality of layers. Layer 24 is a GaN layer while layer 22 comprises AlGaN. As is well known in the art, formation of a high mobility two-dimensional electron gas at the interface between the GaN layer 24 and the AlGaN layer 22 provides the basis for current flow within the diode and in particular can allow for low on-state resistance. The substrate also has a plurality of stress relieving layers shown generally at 26. The stress relieving layers 26 are provided to reduce stress within the substrate 20 associated with the fact that the lattice parameters of the GaN layer 24 are typically different to those of the material forming the underlying region of the substrate 20, including the backside of the substrate shown generally at 28.
The device further includes a semiconductor package that comprises a lead frame 2 and an encapsulant (which is indicated schematically by the dashed line labelled 30 in FIG. 1B). The backside 28 of the substrate 20 is attached to an electrically conductive (typically metallic) part 2A of the lead frame 2.
The Schottky diode has a cathode 6 and an anode 8. A group of bond wires 16 are used to make connections to the cathode 6, and another group of bond wires 18 are used to make connections to the anode 18. In particular, the cathode 6 of the device is connected by the bond wires 16 to the electrically conductive part 2A of the lead frame 2, and the anode 8 of the device is connected by the bond wires 18 to another electrically conductive part 2C of the lead frame 2. The electrically conductive parts 2A and 2C may be connected to output pins of the package (this is represented schematically in FIG. 1B by the portions of the conductive parts 2A and 2C that extend out from the encapsulant 30). Note that electrically conductive parts 2A and 2C are electrically isolated from each other by an isolation region 2B, which may comprise a dielectric material. Bond wires 18 extend across the isolation region 2B to connect the anode 8 to the electrically conductive part 2C. The encapsulant 30 can be used to secure and protect the substrate 20 and the bond wires 16, 18, as is well known in the art.
In use, the voltage on the anode 8 switches between a large negative voltage (with respect to the cathode voltage, which is normally grounded) in a non-conductive state and a low, positive anode-voltage, which causes a large forward current to flow in an on-state of the device.
As noted above, the substrate 20 shown in FIG. 1A is normally attached to the electrically conductive part 2A of the lead frame, such that the backside 28 of the substrate 20 is in electrical communication with the conductive part 2A. Since the cathode 6 of the device is also connected to the conductive part 2A of the lead frame 2 by the bond wire 16, it follows that the cathode 6 of the device is in electrical communication with the backside 28 of the substrate 20. The connection between the conductive part 2A of the lead frame 2 and the cathode 6 can reduce or prevent electromagnetic radiation associated with switching of the anode 8, which turn may otherwise cause electromagnetic interference (EMI).